Expansion of Universe decelerated 11bn years ago

Using a new technique for measuring the three-dimensional structure of the distant Universe, astronomers from the Sloan Digital Sky Survey (SDSS-III) have made the first measurement of the cosmic expansion rate just three billion years after the Big Bang.

For the past five billion years, the expansion of the Universe has been speeding up, powered by the mysterious repulsive force known as dark energy. But the new measurements showed that the expansion of the Universe was slowing down 11 billion years ago.

“If we think of the Universe as a roller coaster, then today we are rushing downhill, gaining speed as we go,” said Nicolas Busca of the Laboratoire Astroparticule et Cosmologie of the French Centre National de la Recherche Scientifique (CNRS), one of the lead authors of the study. “Our new measurement tells us about the time when the Universe was climbing the hill -- still being slowed by gravity,” he stated.

The new measurement is based on data from the Baryon Oscillation Spectroscopic Survey (BOSS), one of the four surveys that make up SDSS-III. It utilizes a technique pioneered by the SDSS in 2005 called baryon acoustic oscillations (BAO). The new measurement does not look at galaxies at all. Instead, it makes use of the clustering of intergalactic hydrogen gas in the distant Universe. SDSS-III researchers’ new measurement of the BAO peak, combined with measurements of the same peak at other points in the Universe’s history, paints a picture of how the Universe has evolved over its history.

The BOSS measurements show that the expansion of the Universe was slowing down 11 billion years ago due to the mutual gravitational attraction of all of the galaxies in the Universe -- but that as the Universe expanded, the constant repulsive force of dark energy began to dominate as matter was diluted by the expansion of space.

“No technique has ever been able to probe this ancient era before,” said BOSS principal investigator David Schlegel of the Lawrence Berkeley National Laboratory. “Back then, the expansion of the Universe was slowing down; today, it’s speeding up. How dark energy caused the transition from deceleration to acceleration is one of the most challenging questions in cosmology,” he added.

Nicolas Busca summarizes: “It looks like the roller coaster crested the hill just about seven billion years ago, and we’re still going.”

The results were presented in a paper submitted to the journal Astronomy and Astrophysics.